1) Pour about 600ml water into a saucepan large enough to take the whole cauliflower. Cover and bring the water to a boil over high heat. Add the cauliflower to the boiling water. Reduce the heat to medium. Cover and cook until a skewer inserts easily into the core of the cauliflower, about 8 minutes. Drain the cauliflower and transfer to the center of a large serving platter. Set aside to cool completely.
2) Arrange the giardiniera, corn and roasted bell peppers in piles around the cauliflower. Sprinkle the onions, olives and capers over the vegetables.
3) Whisk the oil, vinegar, and black pepper in a medium bowl to blend. Drizzle the vinaigrette over the vegetables and serve.
The salad can be prepared ahead. Cover the salad and refrigerate it overnight. Bring to room temperature before serving.